a) Stylistic option, I suppose. Simple ablative seems to me quite lighter than a more verbose quam + case of the antecedent.

b) I don't think it's either nominative or accusative. Instead, the case of the antecedent (which, surely, is more often than not one of those). Puer palÃ¡ minore quam sole ludit. The boy plays with a ball smaller than the Sun.

PS: Refer to Allen 407 (page 265 in the .pdf), for a more accurate and detailed explanation.